This invention relates to a bearing assembly for supporting a shaft for rotational displacement. More particularly this invention concerns a tilt pad journal bearing with regulated lubricant flow to and from each bearing. The pad has an anit-spragging design and may have distinct operating configurations at different temperature ranges.
Many turbomachines having large shafts are supported with a tilt pad type bearing. The tilt pad bearing includes a series of spaced arcuate pads lined with a material such as babbit which when lubricated with oil or a similar lubricant act to maintain a shaft in the desired position while provding little restraint against rotational displacement. The typical tilt pad journal bearing includes a retainer structured to maintain the various tilt pads in position. The tilt pads are arranged such that they may pivot about a point to securely fit against the shaft and such that lubricant may be forced through the bearing to both reduce friction between the bearing surface and the shaft and to cool the bearings.
With a prior art device operating under high temperature conditions the lubricant being flooded through the entire bearing becomes warm as does the bearing. Under certain operating conditions the temperature of the entire bearing becomes sufficiently hot that the life of the bearing is significantly reduced and failures may occur. Additionally the tilt pad bearing which is capable of pivoting may have a portion of its contact surface with the shaft displaced such that it digs into the surface of the shaft creating additional frictional forces. This pivoting such that the edge of the bearing groups or engages the shaft surface is called spragging.
It has also been found that tilt pad bearings should be configured differently when operating under different conditions. When the shaft is rotating very slowly it is desirable to have the shaft surface and the inner surface of the bearing coordinately mate over the entire bearing length and to have a certain amount of oil or lubricant weep into this interface to provide a reduced friction layer for promoting free rotational displacement. This is known as boundary lubrication.
When operating at high speeds it is necessary to utilize dynamic lubrication. Under high speed conditions it is desirable to have the inner surface of the bearing partially mate with the shaft and partially be displaced from the shaft. The desired configuration is to have the leading edge of the bearing displaced from the shaft sufficiently to allow a lubricant wedge to be developed therebetween and the trailing edge of the bearing to mate with the shaft to cause the lubricant to be forced therebetween. The establishment of the lubricant wedge between the shaft and the bearing acts to effectively distribute lubricant over the shaft surface under high speed and potentially high temperature conditions.